1. Field of the Invention
The present invention generally relates to a technique for controlling a motor, and more particularly, to a method for controlling a motor, a circuit for controlling a motor and a brushless motor system using the method or employing the circuit.
2. Description of Related Art
A variable speed motor has been broadly used in many applications, such as a factory automation system, a ventilating system, an air conditioning system and the like. A conventional variable speed motor is a brushed motor, on which a set of brushes and a commutator are disposed. Over the past century, graphite brushes and a slip ring (commutator) which the graphite brushes come to contact with have been used to achieve the commutation of a brushed motor. However, the above-mentioned brushed motor has many drawbacks, it is cumbersome, accompanied by noise during working and inefficient. Along with the progress of since and technology, a novel brushless motor has been developed; and, the improved design and the rapid development in material technology thereof have most probably benefited from the great advancement, the price thereof has been dramatically reduced. Today, the cost difference between a brushed motor and a brushless motor is 10% only, so that a certain trend has appeared that a brushed motor is gradually replaced by a brushless motor.
FIG. 1 is a structure diagram of a typical brushless motor. Referring to FIG. 1, the motor includes three stator coils 11,12 and 13, a rotor 14 employing permanent magnets, Hall sensors 15,16,17 and a circuit for driving a motor 18, wherein all of a terminal of each the stator coil are coupled together, while another terminals (nodes A, B and C) thereof are respectively coupled to the circuit for driving a motor 18 (via three input terminals A, B and C of the circuit 18). The motor is referred to as a three-phase motor as well. The circuit for driving a motor 18 detects the rotation position of the rotor 14 mainly by Hall sensors 15, 16 and 17 and then drives the three stator coils 11-13 via the nodes A, B and C to control the rotation speed and rotation position of the rotor and the like.
In some circumstances or application structures however, a motor does not allow disposing Hall sensors; for example, the compressor motor of a cooler or a refrigerator is a closed motor, which is required to operate at quite high temperatures. Based on the above-mentioned facts, many developers have made efforts to detect the rotation position of a rotor without using Hall sensors. During spinning of a brushless motor's rotor, since the armature where the rotor is mounted on adopts permanent magnets, a back electromotive force (back-EMF) is produced. FIG. 2 is a diagram showing the circuit for driving a motor 18 in FIG. 1 and the equivalent circuit of the conventional brushless motor of FIG. 1. Referring to FIG. 2, the circuit for driving a motor 18 includes transistor switches S201-S206, while the stator coils 11-13 respectively include an equivalent inductance LS, an equivalent resistor in series connection RS and a back-EMF voltage source EA, EB or EC. The circuit for driving a motor 18 respectively controls the on/off state of each of the transistor switches S201-S206 in pulse-width-modulation mode (PWM mode), so as to control the currents ia,ib,ic of the stator coils 11-13. Each of the switches S201-S206 respectively has a PWM-on state and a PWM-off state.
FIG. 3 is a diagram where ideal current waveforms of the conventional three-phase motor shown by FIG. 1 and the back-EMF waveforms generated during the conventional motor spins are schematically illustrated. Referring to FIG. 2, ia,ib,ic herein respectively indicate the ideal current waveforms of the currents fed to the nodes A, B and C in FIG. 1, ea,eb,ec respectively represent the back-EMFs of the stator coils 11-13 and Te represents motor torque. It can be seen from FIG. 3, in order to keep the torque Te constant, under the ideal condition the currents ia,ib,ic should be respectively supplied to the stator coils 11-13 at the 30 electrical degree after the back-EMFs ea,eb,ec respectively cross the zero voltage. In other words, as long as the zero-crossing points of the back-EMFs ea,eb,ec are detected, the position of the motor relative to the zero-crossing points or the preferred time point to supply the currents is obtained, so that the switching time point for properly controlling the switches S201-S206 can be determined; but the back-EMFs ea,eb,ec are unable to be directly measured.
To solve the above-mentioned problem, a scheme was proposed wherein a voltage-dividing circuit is used to measure the PWM voltages of the nodes A, B and C where no current is flowing through and a filter is used to filter the measured PWM voltages, so as to extract the information of the back-EMFs ea,eb,ec. However, it is noted that the scheme by using a voltage-dividing circuit would decay the captured signal. On the other hand, a phase delay of the filter is varied with the motor speed, which makes the detection of a zero-crossing point inaccurate and results in thereby a commutation error of the motor.
References listed below are some patents, in which the conventional methods of controlling a motor without Hall sensors by detecting zero-crossing points to obtain motor positions are given. In the following, the major ideas of the patents [1]-[10] including the drawbacks thereof are briefed. FIG. 4 is a conventional circuit diagram cited from the reference [1]. Referring to FIG. 4, in the patent [1], a clamping circuit 40 coupled to the nodes A, B and C of the stator coils of a motor is used to estimate the voltage at the node B when a current flows from A to C and an upper bridge switch S401 is cut off, the drawback of the method is that a minimum switch closing time is definitely required which limits the application of the method. In the patent [2], an offset compensation circuit is provided to compensate the voltage drop caused by the body-diode in a switch. Although the offset compensation is able to improve the asymmetry of zero-crossing signal. However, the voltage drop of a body-diode varies with the forward current flowing therethough. A compensation of a constant voltage proposed by [2] would cause a commutation phase error. In addition, the disclosed circuit can be used to detect a back-EMF in a PWM-off state only, not in a PWM-on state.
In order to improve the accuracy of detecting a zero-crossing event, several methods and circuits are provided by [3]-[6]. In the patent [3], a comparator is used to override the PWM signal and drives a high-side transistor to conducting state until the zero crossing point is detected. It needs to be aware that the current adjustment ability and the generated ripple component of the motor torque would be affected by overriding the PWM signal. The patent [4] discloses a sampling and holding circuit for sampling back-EMF signals in a PWM-on state and holding the sampled voltage by a capacitor in a PWM-off state. The capacitor herein is discharged through a current source during a PWM-off period, thus, the voltage on the capacitor is substantially increased/decreased with the back-EMF, however, the capacitor and the current source need to be adjusted according to the parameters of a motor. In the patent [5], the current flowing paths of a commutator are modified, wherein an extra power transistor is disposed between a low-side transistor and the grounded terminal. The method by modifying the current flowing paths, however, is not suitable for common applications.
The patent [6] proposes a scheme by estimating the variation between a plurality of different back-EMFs rather than estimating a single back-EMF to advance the signal/noise ratio, wherein the currents of any two phases must be detected in current mode, or the difference between any two phase voltages must be detected in voltage mode, and the application of the method would be limited thereby. The method proposed by the patent [7] is based on a time difference between the previously estimated two zero-crossing times to forecast a next zero-crossing time. However, the method is unable to suit a motor having large speed variations or an asymmetrical motor.
The patent [8] provided a polarity detector for measuring a back-EMF and the patent [9] provided an edge detector for detecting a zero-crossing point are intended to solve the problem that the back-EMF during a conducting period of the body-diode in a switch is unable to be detected. However, it is noted that under some situations, in particular, under a heavy load condition, the polarity of the signal corresponding to a real zero-crossing event and measured by using the method of the patents [8, 9] may not be changed. Referring to the experimental waveforms in FIG. 5, the waveform 501 is the voltage of the node A, the waveform 502 is the current of the node A, the waveform 503 is the waveform detected by the patents [8, 9], and 504, 505 and 506 are respectively indicate voltage variations of the waveform 503. It can be seen from FIG. 5 that the voltages of the waveform 503 at the time points 504 and 506 take a low level, which means a zero-crossing point is detected, respectively; but at the time point 505, the zero-crossing point fails to be measured due to an excessive load so that the waveform 503 still keeps a high level. If a real zero-crossing point fails to be measured by using the above-mentioned schemes of the patents [8, 9], the motor may stop down immediately due to no available commutation, which causes a reverse surge current, burns the pre-stage circuit and puts the motor and the applied system in danger.
The patent [10] provides a method by using a storage unit to record the time history of zero-crossing events, wherein a longest delay time, a middle delay time and a shortest delay time of commutation after a zero-crossing event occurs are recorded to judge whether a commutation sequence advances. The commutation is conducted not at the 30 electrical degrees after a zero-crossing point, but by using a time delay. Therefore, the stored longest, middle and shortest delay time of commutation must be adaptively adjusted according to the motor speed. However, there is no standard to define the longest time and the shortest time; thus, the simplest way to utilize the method provided by the patent [10] is to measure various delay time of commutation in respond to different operation speeds and store the measured time information in the above-mentioned storage unit, which results in cost-wasting a lot.    [01] J. M. Bourgeois, J. M. Charreton, P. Guillemin, and B. Maurice, “Control of a brushless motor,” U.S. Pat. No. 5,859,520, STM, Jan. 12, 1999.    [02] J. Shao, D. C. Nolan, K. A. Haughton, and T. L. Hopkins, “Circuit for improved back EMF detection,” U.S. Pat. No. 6,633,145, STM, Oct. 14, 2003.    [03] E. C. Lee, “BEMF crossing detection in PWM mode operation for sensorless motor control application,” U.S. Pat. No. 5,789,895, STM, Aug. 4, 1998.    [04] R. Sakti and K. K. Chow, “BEMF zero-crossing detection system of a multiple-phase motor,” U.S. Pat. No. 5,909,095, STM, Jun. 1, 1999.    [05] P. Menegoli, “Circuit for improving back-EMF detection in pulse-width modulation mode,” U.S. Pat. No. 5,866,998, STM, Feb. 2, 1999.    [06] G. Maiocchi and M. Viti, “Reconstruction of BEMF signals for synchronizing the driving of brushless sensorless motors by means of predefined driving signals,” U.S. Pat. No. 5,838,128, STM, Nov. 17, 1998.    [07] S. W. Cameron, “Method and apparatus for resynchronizing a moving rotor of a polyphase DC motor,” U.S. Pat. No. 5,172,036, STM, Dec. 15, 1992.    [08] W. S. Gontowski, “Sensorless motor driver with BEMF mask extender,” U.S. Pat. No. 6,504,328, STM, Jan. 7, 2003.[09] Q. Jiang and C. Bi, “Method to detect the true zero-crossing points of the phase back EMF for sensorless control of brushless DC motors,” U.S. Pat. No. 6,879,124, Apr. 12, 2005.[10] B. Maurice and J. M. Charreton, “Control of a brushless motor,” U.S. Pat. No. 6,577,085, STM, Jun. 10, 2003.